Fuel cell systems that generate electricity using hydrocarbon fuel typically comprise a plurality of individual fuel cells, each having an anode side and a cathode side, connected in series or parallel to form a fuel cell stack.
FIG. 1 illustrates one example of a conventional fuel cell system comprising a plurality of tubular solid oxide fuel cells (SOFCs) 10, each having an internal anode 12 and an external cathode 14. The tubular fuel cells 10 are located within a fuel cell main body 11.
During operation of the illustrated fuel cell system, at the cathode side (air side) air is supplied to the cathodes 14 through an air inlet 16 and associated air plenum 17 and the air is heated via a heat exchanger 18 before it reaches the air plenum 17. The heated air passes over the cathodes 14 where oxygen is extracted and some waste heat is dissipated. The waste heat is recovered by the heat exchanger 18 as the oxygen-depleted air is exhausted through an air outlet 20, to thereby heat the air supplied through the air inlet 16. At the anode side (fuel side) of the fuel cell system, a suitable hydrocarbon fuel (for example natural gas containing methane) is injected into a steam reformer 22 which is located externally of the fuel cell main body 11, and hence externally of the fuel cell stack. Steam-reformed hydrocarbon fuel containing hydrogen and carbon monoxide is supplied from the external steam reformer 22 to a fuel plenum 24 which supplies the fuel to the internal anodes 12 of the fuel cells 10 through fuel injection passages 25. The steam-reformed fuel is reacted at the surface of each internal anode 12 such that the hydrogen and carbon monoxide is electrolytically oxidized by oxygen ions passing through the fuel cell surface (comprising cathode, electrolyte and anode layers). Exhaust products containing steam, carbon dioxide and unreacted hydrocarbon fuel are formed at the anode side of each fuel cell 10 and are collected in an exhaust plenum 26. A proportion of the exhaust products from the exhaust plenum 26 are recirculated to the external steam reformer 22 via an external ejector 28, which simultaneously injects fresh hydrocarbon fuel into the external steam reformer 22. The recirculated exhaust products provide the steam that is needed for the steam reforming process and the recirculated unreacted fuel increases fuel utilization and, hence, efficiency. The exhaust products that are not recirculated to the external steam reformer 22 may be used to heat the air supplied through the air inlet 16 or may alternatively be vented. The fuel cell stack is typically operated at a temperature in the order of 700 to 1000° C.
Another example of a conventional fuel cell system comprising tubular solid oxide fuel cells (SOFCs) having an alternative configuration with an internal cathode and an external anode is described in WO 2007/139583 (Siemens Power Generation, Inc.) with particular reference to FIG. 1 of that document. The fundamental operating principles of the external anode fuel cell system described in WO 2007/139583 are the same as those of the conventional internal anode fuel cell system described above with reference to FIG. 1 of this specification.
During operation of a fuel cell system, both endothermic and exothermic reactions take place and thermal management is, therefore, important. The conventional fuel cell systems described above do not provide optimal thermal management and it would, therefore, be desirable to provide a fuel cell system with improved thermal management.